Phosphonium catalyst system for the polymerization of 2-pyrrolidone

ABSTRACT

The process of making a catalyst for the polymerization of 2-pyrrolidone by contacting a phosphonium halide, an alkali metal pyrrolidonate and carbon dioxide in a mol ratio of about 1:0.1-2:0.1-0.5 and the polymerization of 2-pyrrolidone with said catalyst. The alkali metal pyrrolidonate may be prepared by reacting an alkali metal hydroxide with 2-pyrrolidone. The polymerization of 2-pyrrolidone in the presence of ths catalyst system produces poly-2-pyrrolidone of high molecular weight.

This is a division of application Ser. No. 724,802, filed Sept. 20,1976, now U.S. Pat. No. 4,100,146.

BACKGROUND OF THE INVENTION

Poly-2-pyrrolidone is produced by the alkaline-catalyzed polymerizationof 2-pyrrolidone. The catalyst system may comprise a partiallycarbonated pyrrolidonate salt made, for example, by reacting an alkalimetal hydroxide with 2-pyrrolidone, or by reacting a quaternary ammoniumhydroxide with 2-pyrrolidone, dehydrating, and contacting the productwith carbon dioxide (U.S. Pat. No. 3,721,652). Japanese Patent No.47-26195 discloses a process for making a catalyst by reacting anon-water-forming alkali metal compound with 2-pyrrolidone andcontacting the product with a quaternary ammonium halide under anhydrousconditions.

BRIEF SUMMARY OF THE INVENTION

A catalyst capable of producing a high-molecular-weightpoly-2-pyrrolidone is made by contacting certain phosphonium halides, analkali metal pyrrolidonate and carbon dioxide in the mol ratio of1:0.1-2:0.1-0.5.

DESCRIPTION OF PREFERRED EMBODIMENTS

The catalyst of the present invention is capable of producingpoly-2-pyrrolidone having a weight average molecular weight in excess of600,000. The catalyst does not require an anhydrous source of alkalimetal pyrrolidonate. The pyrrolidonate may be made by contacting analkali metal hydroxide with 2-pyrrolidone, rather than by contacting itwith an alkali metal or alkali metal alkoxide.

Catalyst System

In the process of the present invention a catalyst for thepolymerization of 2-pyrrolidone is made by contacting an alkali metalpyrrolidonate, certain phosphonium halides and carbon dioxide in molratio of about 1:0.1-2:0.1-0.5, preferably in mol ratio of about1:0.2-1.5:0.1-0.5, and most preferably in a mol ratio of about 1:1:0.3.

The reactants, the pyrrolidonate, the halide and carbon dioxide, may becontacted in any order beginning with pyrrolidonate as one of thecomponents. It is preferred, but not necessary, to add the alkylphosphonium halide to the previously carbonated pyrrolidonate salt. In apreferred embodiment, the catalyst of the present invention is formed ina solution of 2-pyrrolidone. An alkali metal hydroxide is added to anexcess of 2-pyrrolidone, with which it reacts to produce a solution ofthe alkali metal pyrrolidonate and water in 2-pyrrolidone. The solutionis dehydrated until it contains less than about 0.1-0.2 weight percentwater. Then carbon dioxide is added in the required mol ratio to thepyrrolidonate in the solution at a temperature of about 25° C. Thephosphonium halide is also added in the required mol ratio to thepyrrolidonate at a temperature of about 25° C.

The phosphonium halide is a compound of the formula ##STR1## wherein Xis a halide, preferably chloride, bromide or iodide, and R¹ -R⁴ may bethe same or different hydrocarbyl groups, preferably alkyl groups.

The phosphonium halide is preferably a tetraalkyl phosphonium halide,although alkylaryl phosphonium halide, such as phenyl trimethylphosphonium halide and benzyl triethyl phosphonium halide, may beincluded. The tetraalkyl phosphonium halide is preferably a tetra (C₁-C₆) alkyl phosphonium halide, and most preferably a tetra (C₁ -C₄)alkyl phosphonium halide. The alkyl groups in tetraalkyl phosphoniumhalide may be the same or different. The tetraalkyl phosphonium halideis preferably a chloride, bromide or iodide, most preferably a bromide.The phosphonium halide may be used as a combination of species, e.g., asa mixture of tetramethyl phosphonium chloride and tetraethyl phosphoniumbromide. The phosphonium halide should be substantially soluble underthe alkaline conditions of catalyst system preparation andpolymerization in order to show an appreciable effect on thepolymerization reaction.

The alkali metal pyrrolidonate is preferably sodium or potassiumpyrrolidonate. For certain purposes, it may be advantageous tosubstitute for pyrrolidonate in whole or in part an alkali metalcaprolactamate or the alkali metal salt of another low-molecular-weightlactam, but this is normally not preferred to the use of thepyrrolidonate. The alkali metal pyrrolidonate is preferably made bycontacting the alkali metal hydroxide with excess 2-pyrrolidone, butother methods may be chosen, such as by reacting 2-pyrrolidone with analkali metal or an alkali metal alkoxide. While it is preferable tocontact the phosphonium halide, the pyrrolidonate and carbon dioxide ina 2-pyrrolidone solution, inert solvents may be used in whole or in partto replace the 2-pyrrolidone. Sulfur dioxide is believed to be aninferior substitute for carbon dioxide, but its use is not barred in thepractice of the present invention.

In the catalyst system of the present invention, polymerizationinitiators and polymerization accelerators may also be used.Unexpectedly rapid polymerization to poly-2-pyrrolidone ofsatisfactorily high molecular weight is achieved in this catalyst systemby the addition of 0.05-1.5 mol percent of an N-acyl lactamateaccelerator such as that which is believed to be produced by thereaction between 2-pyrrolidone or caprolactam and acetic acid anhydride.Preferably 0.05-0.5 and most preferably about 0.1-0.2 mol percent of anN-acyl pyrrolidone is used. The preferred N-acyl pyrrolidone is N-acetylpyrrolidone.

Polymerization Conditions

The polymerization process of this invention is specifically applicableto the polymerization of 2-pyrrolidone to form a polymeric carbonamideof high molecular weight in a reasonable polymerization time of 8-24hours. Weight average molecular weights in excess of 600,000 have beenattained. The high-molecular-weight polymer is capable of being formedinto filaments having substantial orientation along the filamentaryaxis, high tensile strength and other properties suitable for makinginto textiles. It can be made into shaped articles and film bymelt-molding or extrusion.

In order to produce high-quality poly-2-pyrrolidone capable of beingformed into fibers, filaments and yarn of commercial textile quality, itis necessary that the 2-pyrrolidone monomer be of high purity. Dependingupon the process of manufacture, commercially available 2-pyrrolidonemay contain appreciable amounts of various impurities, some of which arebelieved to interfere deleteriously with polymerization. Purification ofthe monomer to polymerization grade is achieved by known purificationtechniques, including distillation.

The process of the present invention is just as applicable to theproduction of polymers of C-alkyl-substituted pyrrolidone, such as4-methyl-2-pyrrolidone and copolymers of 2- pyrolidone, such as withcaprolactum, as to the production of poly-2-pyrrolidone.

Preferably the catalyst system comprises about 0.5-30 mol percent ormore of the 2-pyrrolidone-catalyst mixture, based on total2-pyrrolidone, preferably about 5-20 mol percent, and most preferablyabout 10 mol percent catalyst. Total 2-pyrrolidone consists of2-pyrrolidonate catalyst, including alkali metal pyrrolidonate andphosphonium pyrrolidonate, as well as carbonated alkali metalpyrrolidone and carbonated phosphonium pyrrolidonate, and 2-pyrrolidoneprovided as solvent to said catalyst, and any additional monomer chargedto the mixture for polymerization reaction. The polymerization catalystsystem is believed to comprise principally phosphonium pyrrolidonate andcarbonated phosphonium pyrrolidonate, but substantial amounts of alkalimetal pyrrolidonate and carbonated alkali metal pyrrolidonate(carboxypyrrolidonate) may also be present, depending upon the molratios chosen. Alkali metal halide is known to be present, but it isbelieved to be inert towards the polymerization reaction. Polymerizationinitiators or activators, such as N-acyl derivatives of the lactams, orcompounds forming N-acyl derivatives of 2-pyrrolidone, may also be addedto the catalyst system as an aid to polymerization.

In general, 2-pyrrolidone may be polymerized at a temperature from about15° C. to about 100° C., preferably 25° C. to 70° C., and mostpreferably from about 40° C. to about 60° C., under a pressure rangingfrom subatmospheric to superatmospheric, in the presence of the catalystsystem for a period from about 4 to about 100 hours or longer,preferably for about 8 to about 72, hours, and most preferably fromabout 8 to about 48 hours. In continuous operation, polymerization timerefers to average residence under polymerization conditions. A smallamount of water, not exceeding about 0.1-0.2 weight percent, based ontotal 2-pyrrolidone, is permissible in the reaction mixture, but lessthan 0.1 weight percent is preferred.

Preparation of polymers of 2-pyrrolidone, according to the normalprocess of this invention, can be carried out with various amounts ofmonomers, catalyst, inert nonsolvent liquids, initiators and otheradditives--the amount of each being properly coordinated to produce themost effective polymerization. Although the preferred amounts of thecomponents in the reaction have been given, it is understood that theseare not intended to be limitations to polymerization, since it may bepossible to achieve substantial polymerization outside the preferredranges.

                  TABLE                                                           ______________________________________                                                       %                                                              % R.sup.1 R.sup.2 R.sup.3 R.sup.4 PX.sup.1                                                   Conversion.sup.2                                                                           Mw × 10.sup.-3                              ______________________________________                                        (C.sub.4 H.sub.9).sub.4 PBr                                                                  45.3         635                                               (C.sub.4 H.sub.9).sub.4 PCl                                                                  18.8         100                                               ______________________________________                                         .sup.1 Mol ratio of potassium to phosphonium halide about 2.                  .sup.2 5 mol percent KOHderived potassium pyrrolidonate (30 mol % CO.sub.     added). Polymerized 22 hours at 50° C.                            

The table is illustrative of the polymerization of 2-pyrrolidoneobtainable from the phosphonium halide catalyst systems of the presentinvention. Catalyst preparation and polymerization were performed ordescribed above. Other than carbon dioxide, no polymerizationactivations or initiators were used. Polymerizations using other oniumsalts, performed under substantially the same conditions as thosedescribed in the table, show that the phosphonium halides are among thefew onium salts which can be successfully employed to providesatisfactory catalyst systems for 2-pyrrolidone polymerization. Forexample, attempts to utilize (CH₃)₃ HNCl, (C₆ H₅)₃ CPF₆, (CH₃)₃ OPF₆,and C₆ H₅ N₂ PF₆, as substitutes for the phosphonium halides of thetable, failed to produce appreciable polymerization of 2-pyrrolidone.

What is claimed is:
 1. The process of polymerizing a 2-pyrrolidonecomposition by contacting an alkali metal pyrrolidonate, a tetraalkylphosphonium halide and carbon dioxide in a mol ratio 1:0.1-2:0.1-0.5,respectively, and additionally 0.01-0.5 mol percent of N-acylpyrrolidone or N-acylcaprolactam or acetic anhydride, with 2-pyrrolidonemonomer to yield a poly- 2-pyrrolidone polymer capable of being formedinto filaments, films, or shaped articles.
 2. The process according toclaim 1 wherein said N-acyl pyrrolidone is N-acetyl pyrrolidone.